The present invention relates generally to processes for purification of plasticizer compositions. More specifically, the present invention relates to processes for purification of epoxidized ester plasticizers.
Such epoxidized fatty acid esters have lately been of considerable interest for use as renewable source-based or—derived plasticizers for various plasticized polymer compositions and end uses, including for alkyd resins, amino resins, epoxy resins (usually phenolic materials), phenolic resins, polystyrene, polyurethane, poly (vinyl alcohol) and unsaturated polyesters. In particular, such materials have been investigated for use in polyvinyl halide compositions.
Polyvinyl chloride (PVC), the most common vinyl halide polymer, finds commercial application in a rigid, substantially unplasticized form and in a plasticized PVC form. Rigid PVC, with which the present invention is not concerned, is used for pipework, ducts and the like in which high chemical resistance is needed but not flexibility or pliability. Plasticized PVC, on the other hand, finds application in films, sheeting, wire and cable coverings, moldings, conveyor belting, toys and hose, in addition to serving as a leather substitute and as a fabric covering for upholstered furniture, automotive seating and other articles.
Broadly speaking, plasticizers are materials which are combined with polymers such as polyvinyl chloride (hereinafter, PVC) to impart flexibility, extensibility and workability or some combination of these attributes to the polymer, as needed for a particular end use. In 1951, the International Union of Pure and Applied Chemistry (IUPAC) developed a universally accepted definition for a plasticizer as a substance or a material incorporated in a material (usually a plastic or an elastomer) to increase its flexibility, workability, or distensibility. Frequently, a combination of primary and secondary plasticizers is used, with the secondary plasticizers not acting in and of themselves to impart the desired attributes to the PVC but serving to improve the effectiveness of the primary plasticizer(s) and optionally offer other characteristics to a PVC composition in which the materials are incorporated.
Historically, the majority of primary PVC plasticizers have been petroleum-derived phthalates and benzoate compounds, dioctyl phthalate and diisononyl phthalate being notable examples. However, such petroleum-derived plasticizers are frequently expensive to produce and use because of fluctuations in the pricing and availability of petroleum, and are increasingly likely to remain so as petroleum reserves are reduced and new supplies prove more costly and difficult to secure. Further, certain of the petroleum-derived phthalate plasticizers have raised concerns for their potential to disrupt human endocrine activity, and regulatory controls have been established in a number of countries to address these concerns.
Unmodified renewable source derived oils, such as triacylglycerol oils, are largely incompatible with PVC resin, but certain modified derivatives of renewable source derived oils, such as epoxidized soybean oil (ESO), are compatible with PVC resin and have been actively investigated for use as a lower cost, renewable source derived alternative to the petroleum-based plasticizers, both as primary and secondary plasticizers. The interest in developing useful plasticizers from renewable sources, such as renewable source derived animal, algal, microbial, or plant (including vegetable) oils, has developed partly also from the expectation that such materials would be less likely to cause physiological disturbances or other injuries to persons coming into contact with products which require plasticizers in their composition. In recent years, as a result, a number of different renewable source-based plasticizers for PVC have been introduced in the literature and in the marketplace. The plasticizer compounds can be used alone or in various mixtures, including many other plasticizers known in the art, such as esters of dicarboxylic acids, citric acid, and the esters of aromatic dicarboxylic acids (e.g., phthalic acid esters). Particularly useful are mixtures comprising plasticizer compounds prepared from epoxidized triglycerides with a high degree of epoxidation for plasticizing PVC. Such epoxidized triglycerides can be typically exemplified by epoxidized soybean oil and epoxidized linseed oil, while other epoxidized renewable source derived oils are also useful. In such formulations, the epoxidized fatty acid fragments provide a desired stabilizing effect by acting as scavengers of acidic polymer decomposition products. The plasticizer compounds are useful to make various industrial and consumer articles, including flooring materials, siding elements for exteriors and interiors of buildings, window frames, flexible and rigid pipes, tubing, reinforced hoses, artificial leather, packaging of consumer articles, interior and exterior automotive parts, electronic equipment cases, various single and multi-layered films, vinyl office supplies, plastisols, and the like.
The purified epoxidized ester plasticizers of the present invention can be made by either transesterification or interesterification and a purification process according to the present invention, for use subsequently as primary or secondary plasticizers in a variety of polymers, including halogenated polymers, acid-functionalized polymers, anhydride-functionalized polymers, and nitrile rubbers. An exemplary halogenated polymer is a PVC polymer, where “PVC” or “polyvinyl chloride” as used herein is understood to cover the range of homo—and copolymers of vinyl chloride with typically up to 20% of co-monomers such as vinyl acetate, propylene, ethylene, diethyl maleate, dimethyl fumarate and other ethylenically unsaturated co-monomers. Examples of other halogenated polymers include polyvinyl halide polymers, chlorinated polyolefins and chlorinated rubbers. Suitable acid-functionalized polymers include acrylic acid-functionalized polymers, as well as acrylic and other polymers in need of plasticization to reduce glass transitions or improve toughness.
Where used as primary plasticizers, the purified epoxidized fatty acid esters can comprise preferably at least 20 percent by weight of a polymer composition, more preferably will comprise at least 30 percent by weight of a polymer composition, and most preferably will comprise at least 50 percent by weight of a polymer composition. A suitable composition can comprise a plastisol.
The resultant plasticized polymer compositions can be formulated, it is noted, in all other respects in a conventional manner, including with incorporating various kinds of additives in addition to the inventive purified epoxidized esters. When the purified epoxidized esters are used in preferred embodiments as the primary plasticizers of a primary/secondary plasticizer system, for example, a renewably-based secondary plasticizer and thermal stabilizer such as epoxidized soybean oil can be added, or other secondary plasticizers (including petroleum-based plasticizers) or other additives for improving one or more properties of heat stability, lubricity or weathering resistance, as ultraviolet absorbers, fillers, anti-oxidants, anti-static agents, anti-fogging agents, pigments, dyestuffs, crosslinking aids and the like can be incorporated in the compositions. The inventive purified epoxidized esters may also be used in certain embodiments in combination with other primary plasticizers such as dioctylphthalate, other phthalates, citrates, benzoates, trimellitates, and other aliphatic diesters, though preferably the plasticized polymer compositions of the present invention will not include any added phthalates and will include substantially only renewably-based or biobased plasticizers.
The epoxidized ester plasticizers to be purified according to the present invention are synthesized by known routes. For example, in commonly-assigned, co-pending U.S. patent application Ser. No. 14/125,602, published Apr. 24, 2014 as US Pat. Appl. Pub. No. 2014/0113999 for “Reduced Color Epoxidized Esters from Epoxidized Natural Fats and Oils”, the contents of which are hereby incorporated by reference in their entirety, reduced color epoxidized fatty acid esters (such as epoxidized methyl soyate, EMS) are made from an epoxidized natural fat or oil (such as epoxidized soybean oil, ESO) through the inclusion of borohydride in either a transesterification process or in an interesterification process.
In another example, in commonly-assigned, co-pending U.S. patent application Ser. No. 14/350,590, published Sep. 4, 2014 as US Pat. Appl. Pub. No. US 2014/0249322 for “Making Epoxidized Esters from Epoxidized Natural Fats and Oils”, the contents of which are hereby incorporated by reference in their entirety, epoxidized esters are made by a transesterification process using low moisture fats and oils. The products undergo phase separation, and reduced molar excesses of alcohol may be employed compared to processes not employing a low moisture feedstock.
In another example, in commonly-assigned U.S. Pat. No. 8,703,849, “Processes for Making High-Purity Renewable Source-Based Plasticizers and Products Made Therefrom”, issued Apr. 22, 2014, the contents of which are hereby incorporated by reference in their entirety, we described processes for making certain high purity unsaturated fatty acid esters from alcohols including 5 to 7 members in a ring structure, whether cyclic, heterocyclic or aromatic in character, which esters could then be epoxidized (according to a second aspect) to yield renewable source-based plasticizers for polyvinyl halide polymers, and in particular, for PVC. The plasticizers could be incorporated easily into PVC as primary plasticizers at even plastisol levels, and provided plasticized PVC compositions in turn that exhibited improved and unexpected performance in certain respects.
While epoxidized benzyl esters of unsaturated fatty acids had been described or suggested previously for plasticizing PVC, see for example, U.S. Pat. No. 3,377,304 (epoxidized benzyl soyate) and British patent No. GB 1,049,100, the known methods of making those benzyl esters and subsequent plasticizers resulted in PVC compositional limitations and performance characteristics which had unfortunately limited the use of such materials only to be secondary plasticizers and thermal stabilizers.
We found that by preparing the indicated unsaturated fatty acid esters (including, of course, benzyl soyate esters), whether by reacting alcohols including 5 to 7 members in a ring structure with unsaturated fatty acid lower alkyl esters having low residual monoglycerides and diglycerides or by reacting the alcohols with an unsaturated fatty acid feed having a correspondingly low monoacylglycerol and diacylglycerol content, these limitations could be overcome.
One very important attribute of plasticizers is the degree of purity of the plasticizer. After plasticizer synthesis, common impurities include metals, ions, monohydric alcohols, such as methanol or benzyl alcohol, and polyhydric alcohols, such as glycerol. Impurities in plasticizers present several problems in the plasticizer and products made therefrom. Impurities may impart odors or colors to plasticizers. Metals and ions are especially problematic plasticizer impurities; they can interfere with the electrical properties of plastics that they are incorporated into, such as their electrical insulating ability. Nonhomogeneity of the polymer crystallite interstices is a result of contaminating ions in plasticized PVC: “The ions move through free spaces which exist in the non-crystalline part between polymer crystallites. The free spaces have nonhomogeneous size distribution depending on a property of polymers or crystallization conditions. The homogeneity or nonhomogeneity of the size of the free space affects the ionic motion, and consequently it also affects the electrical properties of polymer solids.” (“Study of motion of impurity ions making influence on electrical properties of polymer solids in low frequency range,” Anada, Y., Advanced Materials Research, 740, 630-635, 2013 (Trans Tech Publications Ltd.)). In PVC plastisols, the electrochemical potential (zeta potential) of PVC particles is related to plasticizer exudation. (“An attempt to describe nonspecific interactions of polymer-polymer type in the plastisol dispersions of poly(vinyl chloride),” Makarewicz, E., Colloid and Polymer Science 267(9), 803-7, 1989). The zeta potential of PVC particles is in turn influenced by the ionic impurities contained in the particles, which originated from additives. This can be very important in the final product, because the presence of even small amounts of acid or alcohol impurities in plasticizers can reduce the efficiency of the plasticizer. (http://www.plastemart.com/Plastic-Technical-Article.asp?LiteratureID=1602&Paper=essential-compounding-chemicals-used-with-PVC-resin-primary-secondary-plasticiser-heat-light-stabilisers, accessed Jul. 3, 2014).
Another potential source of impurities in epoxidized ester plasticizers is the characteristic oxirane ring. The labile nature of the epoxide functionality in the epoxidized renewable source derived fats or oils contemplated as starting materials for epoxidized esters renders the epoxides susceptible to formation of undesired by-products. This highly reactive ring is easily opened, and shows the typical reactions of ethylene oxide. Oxirane ring compounds are highly sensitive to acid-catalyzed reactions. They are also susceptible to reacting with alcohols, amines, carboxyl groups, imines, phenols, and inorganic cyanates. (“Epoxidation Products of Fatty Acids, Alkyl Fatty Acid Esters and Glycerides, etc. as Chemical Intermediates and Plasticizer/Stabilizers for Polymers, Resins and Rubbers, Part I” Lower, E. S., SOFW Journal 121(4) 278, 280-2 1995).
The problem of by-products arising from hydrolysis or rearrangement of epoxides and from cross-linking of the fatty acid chains is a recognized source of impurities in epoxidized ester plasticizers (U.S. Pat. No. 8,436,042, issued May 7, 2013, the contents of which are hereby incorporated by reference in their entirety).
Therefore, considerable effort is often expended to remove impurities from plasticizers due to the disproportionate negative effect of impurities on the properties of compositions, including PVC plastics, they are incorporated into. Unfortunately, the known approaches to plasticizer purification are cumbersome and costly. Plasticizers are often slightly acidic due to the presence of small amounts of organic acids. Acids in plasticizers, even at low concentrations, attack the plasticizer and will cause discoloration of PVC on exposure to slight heating. Washing (extraction with water) is often carried out on an industrial scale in the reactor vessel used to synthesize a plasticizer. The washing process is expensive, and consumes valuable reactor time that could be used in plasticizer production. Ironically, the water washing also generates large amounts of waste wash water, thus reducing process sustainability by running counter to the very values promoted by the use of renewable source derived plasticizers.
Metal ions and their corresponding salts are notoriously problematic in PVC for causing degradation, including photodegradation, of the PVC. Metal salt contaminants increase the light-sensitivity of PVC by acting as sensitizers of the decomposition of peroxide groups or by generating free radicals under UV irradiation. Typical ionic and metal contaminants in plasticizers include sodium, iron, calcium, phosphorus, zinc, boron, molybdenum, and aluminum.
One example of plasticizer purification is taught in U.S. Pat. No. 3,070,608 (issued Dec. 25, 1962, assigned to Swift & Company), which required extraction of methyl esters of epoxy acids with “a large volume of water” (Example 1, Col 3 line 4); the washing removed “excess alcohol, glycerine, and sodium methylate catalyst” (Example 4, Col 4 lines 20-23. Subsequent drying was also necessary. The use of a large volume of water is believed to be cumbersome and costly.
Another example of plasticizer purification is taught in U.S. Pat. No. 8,802,877 (issued Aug. 12, 2014; assigned to NPC Industrias Quimicas Ltda). In this application, product mixture purification required several steps:                “At the end of the transesterification reaction a purification step occurs through which the formed glycerin is decanted and separated. The reaction mixture is then neutralized with acid, washed and stripped (alcohol removal process), with steam and under a discreet vacuum (up to about 600 mmHg [80 kPa]), at temperatures varying from 120 to 220° C., depending on the type of alcohol which is retrieved and re-used in subsequent production. After the alcohol removal process, the product is filtered.” (Col. 7 lines 4-12)The high-temperature stripping with steam can cause formation of heat-related defects, such as color defects, and the execution of so many steps is believed to be cumbersome and costly.        
Yet another example of plasticizer purification is set forth in European Patent Application Publication No. EP2 070 980 (published Jun. 17, 2009, assigned to Nexoleum Bioderivados Ltda.). After the formation of epoxidized ethyl soyate plasticizer, phase separation was carried out in a separation funnel as follows:                “At the end of the reaction, the water phase is separated from the oil phase in a separation funnel. The oil phase contains the epoxidized ester and the formic acid. The wash of the oil phase is processed according to the procedure described for the wash of the ethyl ester after the transesterification; although the water needs to be preheated to 50° C. and the wash procedure shall be repeated until all the formic acid has been eliminated.” Page 3 lines 47-51The execution of repeated washings with water preheated to 50° C. is believed to be cumbersome and costly.        
Another example of plasticizer purification is taught in United States Patent Application Publication No. US2013/203907 (published Aug. 8, 2013, assigned to Arkema, Inc.). Residual sodium, calcium and/or magnesium ions, contaminants, and detrimental reaction by-products were removed from epoxidized plasticizers with one or two water-washing steps to remove hydrogen peroxide and formic acid. The compositions are then subjected to steam-stripping and drying under full vacuum (Example 1, [0043] of US2013/203907). The high-temperature stripping with steam can cause formation of heat-related defects, such as color defects.
In commonly-assigned United States Patent Application Publication No. US2014/113999, epoxidized methyl soyate having a Pt—Co Hazen color of 90 or less as determined by ASTM D1209 was obtained after neutralization with a solution of citric acid, three or four iterative water washes, drying over anhydrous magnesium sulfate, filtration, and vacuum drying overnight.
In another example of plasticizer purification, double distillation is proposed to remediate odor problems in plasticizers at (http://www.plastemart.com/Plastic-Technical-Article.asp?LiteratureID=1602&Paper=essential-cornpounding-ehemicals-used-with-PVC-resin-primary-secondary-plasticiser-heat-light-stabilisers, accessed Jul. 3, 2014). However, subjecting the plasticizer to double distillation can cause the formation of heat-related defects, such as color defects.
The present invention in one aspect addresses this need, in providing a process for purifying epoxidized ester plasticizer compositions. The process is simple and continuous, and does not require the high temperatures used in distillation.